1. Field of the Invention
The present invention relates to an automatic brake control system for a vehicle in which brake is automatically applied when the vehicle comes close to an obstacle or a vehicle running ahead.
2. Prior Arts
In recent years, systems for avoiding a collision with an obstacle or a preceding vehicle presenting ahead of a vehicle have been developed. Such systems, for example an automatic brake control system includes an intervehicle distance detecting apparatus for detecting a distance between the vehicle and the preceding vehicle based on images taken by cameras, signals from a laser-beam radar or the like and an automatic brake apparatus for applying brake to the vehicle when the intervehicle distance becomes smaller than a threshold distance.
In most cases, such a threshold distance is calculated based on parameters such as a vehicle speed of an own vehicle, a vehicle speed of a preceding vehicle, an assumed deceleration of the own vehicle, an assumed deceleration of the preceding vehicle and the like. For example, Japanese Patent Application Laid-open No. Toku-Kai-Hei 6-144169 discloses a technique in which the threshold distance is calculated from a road friction coefficient, a measured intervehicle distance, vehicle speeds, a maximum deceleration to be produced with the road friction coefficient and a time-versus differential value of the intervehicle distance.
Further, in most cases, such a target deceleration is calculated based on parameters such as a vehicle speed of the own vehicle, a vehicle speed of the preceding vehicle and an intervehicle distance actually detected.
However, generally a vehicle speed sensor for detecting the vehicle speed of the own vehicle, particularly a vehicle speed sensor in which a vehicle speed is calculated from a wheel speed,, has a defect that an extremely low speed, i.e., 2 to 3 km/hour, is difficult to be detected accurately. As a result, a traveling speed of an obstacle (in this case, preceding vehicle) is unable to be calculated accurately, because it is calculated based on the speed of the own vehicle and the intervehicle distance.
Therefore, when the vehicle travels at extremely low speeds, if the threshold distance and the target deceleration are calculated based on the own vehicle speed and the preceding vehicle speed, there is a fear that those threshold distance and target deceleration can not be established accurately and as a result the automatic brake control system does not operate properly.
Further, under the condition that the vehicle is very close to the preceding vehicle, an error in the intervehicle distance detected by the distance sensor can not be neglected. Especially when the vehicle is in the state of almost stopping, the target deceleration produces a large error and as a result, for example, when it is established to an unduly low value, such failures as the braking force is released immediately before stopping may occur.